Plus, there are bigger-picture questions, including whether 3D mammography -- known as digital breast tomosynthesis -- ultimately makes a difference in how breast cancer patients fare, one expert said.

"We would like to see long-term outcomes," said Dr. Sarah Friedewald, the lead researcher on the new study and a radiologist at Advocate Lutheran General Hospital in Park Ridge, Ill.

But she said her team's findings point to a shorter-term advantage. "We found 3D mammography really does help doctors find more invasive breast cancers while cutting down on callbacks," Friedewald said.

A "callback" happens when a mammogram picks up something suspicious, and the doctor wants to do additional imaging or a biopsy. For most women, it turns out to be nothing; according to the American Cancer Society, fewer than 10 percent of women called back for more testing are diagnosed with breast cancer.

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If 3D mammography can reduce those callbacks, "that's a pretty big deal," Pisano said. Additional tests can be anxiety-provoking for women, and they use up time and resources, she said.

In the United States, 3D mammography has been available since 2011. It was originally approved by the Food and Drug Administration to be used along with standard, two-dimensional digital mammograms. But last year, the agency approved a 3D system that can be used alone.

Conventional mammography takes images of the breasts from two angles. In contrast, the 3D scanner moves in an arc over the breasts, taking images from various angles. The point is to improve doctors' ability to spot small tumors and reduce false scares.

One concern, though, is that the greater sensitivity will find more growths called ductal carcinoma in-situ, or DCIS. Those are abnormal cells in the milk ducts that may, or may not, progress to cancer. Since doctors have no way of telling, women with DCIS usually receive treatment.

In the new study, though, rates of DCIS detection didn't rise.

Friedewald's team looked at nearly 455,000 screening mammograms done at 13 hospitals that all switched from digital mammography to digital-plus-3D after the 2011 FDA approval. In the year before switching, the hospitals found 1.4 cases of DCIS per 1,000 screenings, and that remained unchanged after the switch.

Instead, detection of more-advanced, "invasive" cancers went up 41 percent.

"That suggests it's finding more important cancers," Pisano said.

Across the hospitals, doctors caught 5.4 cancers for every 1,000 women screened -- versus 4.2 per 1,000 in the year before the centers added the 3D technology.

Still, Pisano said the best proof that 3D is actually better would come from a clinical trial, done at multiple hospitals, where women are randomly assigned to either standard mammography or 3D.

"I do believe we should prove 3D mammography -- the newest [stand-alone] version -- is better than digital before everybody goes out and buys one of these machines," Pisano said.

One question, said Pisano and Friedewald, is whether 3D might work best for certain women -- such as younger women with denser breast tissue.

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"We haven't figured it all out yet," Pisano said.

But, she added, after years of "one-size-fits-all" mammography screening guidelines, research may lead to more tailored recommendations for different groups of women.

Hologic, Inc., of Danbury, Conn., which manufactures a 3D mammography scanner, funded the study. Friedewald and several co-researchers are advisors to the company.